1. Field of the Invention
This invention relates to a rear projection display device for enabling an observer to observe a picture on a front surface of a screen by slantly projecting image light on a back surface of the screen.
2. Description of the Prior Art
FIGS. 9, 10 illustrate one example of a conventional rear projection display device. FIG. 9 is a cross sectional view schematically illustrating a structure of a conventional rear projection display device, and FIG. 10 is a top plan view schematically illustrating a structure of a projection unit of the rear projection display device of FIG. 9. In the following description, a coordinate system is used where a horizontal direction of a rectangle screen 170 is taken along an x-axis, a vertical direction of the screen 170 is taken along a y-axis, and a perpendicular direction to the screen 170 is taken along a z-axis.
As shown in FIG. 9, the rear projection display device includes a projection unit 120 arranged in a body 110. A projection lens 130 is arranged on a light emitting opening of the projection unit 120. A reflecting mirror 160 is arranged on an inner back surface of the body 110, and a transmission type diffusing screen 170 is arranged on the front of the body 110. Image light which is magnified and projected from the projection unit 120 through the projection lens 130 is reflected on the reflecting mirror 160, is irradiated onto a back surface of the diffusing screen 170, and then a picture is observed on the front surface of the diffusing screen 170.
As shown in FIG. 10, the projection unit 120 includes a white light source 121 comprising a lamp 121a and a reflector 121b. Dichroic mirrors 122, 123 split white light emitted from the white light source 121 into light of three colors. A first dichroic mirror 122 selectively reflects light of a red component (referred as red light hereinafter) out of the white light emitted from the lamp 121a and transmits light of other color components. A second dichroic mirror 123 selectively reflects light of a green component (referred as green light hereinafter). Green light out of the light which is transmitted through the first dichroic mirror 122 is selectively reflected on the second dichroic mirror 123 and is introduced to a liquid crystal panel 127g for green. Light of a blue component (referred as blue light hereinafter) out of the light transmitted through the second dichroic mirror 123 is introduced to a liquid crystal panel 127b for blue by reflecting mirrors 125, 126. The red light reflected on the first dichroic mirror 122 is introduced to a liquid crystal panel 127r for red by the first reflecting mirror 124.
Color light respectively modulated at the liquid crystal panels 127r, 127g, and 127b is synthesized at a dichroic prism 128 and is emitted to the projection lens 130.
Incident directions of the color light modulated at the liquid crystal panels 127r, 127g, and 127b to the dichroic prism 128 is set with consideration of color reproducibility at the dichroic prism 128. Light reflected on the dichroic prism 128 is S-polarized light, and light transmitted through the dichroic prism 128 is P-polarized light.
S-polarized light is a linearly polarized light which the oscillation direction of the electric vector of light incident to a sample surface is vertical to a surface including a normal of the sample surface and a normal of wave surface which is a light traveling direction. P-polarized light is a linearly polarized light which the oscillation direction of the electric vector of light incident to a sample surface is included in an incident surface (a surface including a normal of the sample surface and a light traveling direction).
Specifically the red light out of the light incident to the dichroic prism 128 is set to be S-polarized to a bonded surface 128x. A polarized light component which is perpendicular to an x-z plane is reflected on the bonded surface 128x. The green light is set to be P-polarized light to the bonded surfaces 128x, 128y. A polarized light component which is parallel to the x-z plane is transmitted through the bonded surface 128x, 128y. The blue light is set to be S-polarized light to the bonded surface 128y. A polarized light component which is perpendicular to the x-z plane is reflected on the bonded surface 128y. And then the red, green, and blue light is color-synthesized.
The color-synthesized image light is irradiated from the projection lens 130 to the back surface of the screen 170 through the reflecting mirror 160.
Recently a rear projection display device capable of slantly irradiating image light to the screen 170 for reducing the depth of the device has been proposed. When the above mentioned projection unit 120 is used for slantly projecting image, a polarization direction of the projected image light to the screen 170 is set in the direction orthogonal with the polarization direction of the image light to the dichroic prism 128. The red light is P-polarized, the green light is S-polarized, and the blue light is P-polarized.
When the image light is slantly projected to the screen 170, the light is incident to the acrylic resin from an air with a certain angle of incidence out of a vertical incidence. FIG. 6 is a table showing the reflectivity characteristic of light incident to the acrylic resin from the air. As shown in FIG. 6, when the image light is slantly projected on the screen 170, the reflectivity of P-polarized light to the screen 170 is lowered while the reflectivity of S-polarized light to the screen 170 increases.
FIG. 8 presents the spectral luminous efficiency characteristics of a man. As shown in FIG. 8, the spectral luminous efficiency of man's eyes is the highest at around a wavelength of 555 nm which corresponds to a color of green and a man is likely to recognize green light brighter in comparison with red and blue light.
As a result, when the image light is slantly projected by using the conventional projection unit 120, the reflectivity of the brightest green light at the screen 120 increases, and the brightness as a whole is lowered, and furthermore the image quality is degraded because of the reflected light.